1. Field of the Invention
This invention relates to a method of creating a curve for correcting the gradation of a radiation image used for viewing, particularly for diagnostic purposes, and an apparatus for carrying out the method. This invention particularly relates to a method of creating a gradation correction curve used for conducting intermediate processing in a radiation image system wherein a radiation image is read out from a stimulable phosphor carrying the radiation image stored therein and reproduced into a visible image, and an apparatus for carrying out the method.
2. Description of the Prior Art
A novel radiation image system for recording a radiation image in a stimulable phosphor, reading out the radiation image from the stimulable phosphor, and reproducing a visible radiation image on a recording material such as a photographic film is disclosed, for example, in U.S. Pat. No. 4,258,264. This system comprises the steps of having a radiation passing through an object and then absorbed by a stimulable phosphor, stimulating the phosphor by light energy to cause the phosphor to emit the radiation energy stored therein as light, detecting the emitted light and converting it to an electric signal used for reproducing a visible image.
This radiation image system, using the stimulable phosphor, is advantageous over the conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure. This is very advantageous in practical use.
Since X-rays do harm to the human body, it is undesirable from the viewpoint of safety to expose the human body to X-rays of high dose. Therefore, it is desirable that the necessary information in the radiograph be obtainable by exposing the human body only once to X-rays of comparatively low dose. On the other hand, radiographs should preferably have both a wide exposure latitude and high image quality suitable for viewing, particularly for diagnostic purposes. Unfortunately, since the conventional radiography is designed so as to satisfy all the required conditions to some extent, the range of recording density or the ability to record various levels of information is insufficient (one approach to the elimination of this problem is the aforesaid radiation image system using the stimulable phosphor), and the image quality is not sufficient for viewing, particularly for diagnostic purposes. Thus, in the conventional radiography, neither of these requirements is completely satisfied.
To solve the above problems, it has been proposed in U.S. Pat. No. 4,302,672 to improve the image quality, particularly the diagnostic efficiency and accuracy of a radiation image by, when a frontal chest radiation image stored in a stimulable phosphor is reproduced into a visible image on a recording material, conducting an intermediate processing for correcting the gradation of the radiation image so that the contrast of the background of the human body is decreased, thereby increasing the contrast of the heart and the lungs. Namely, in this gradation processing method, density-to-electric signal conversion is conducted to correct the gradation as described above.
In general, when the aforesaid gradation processing method is carried out, a density-to-electric signal conversion pattern (i.e. the gradation correction pattern) adapted to the object and the use purpose is created in advance. On the basis of this pattern, a signal conversion processing is conducted on an electric signal obtained by reading out the radiation image from a stimulable phosphor and converting it photoelectrically, thereby to conduct the gradation processing. The aforesaid gradation correction pattern is normally called the gradation correction curve.
However, in the aforesaid gradation processing method, when the gradation correction curves are created according to various objects and various use purposes, the number of the gradation correction curves becomes about several tens. Therefore, very much manpower is required for gathering the data for creating the gradation correction curves, and an electronic computer or the like having a very large capacity for digitally converting and storing the data. Thus, from the practical viewpoint, this method is not simple.